This invention relates to television apparatus, and more particularly, to a system for providing a television display which portrays the path of motion of a moving object in an otherwise static scene.
In commonly assigned application Ser. No. 864,405 filed Dec. 27, 1977, entitled "Television System for Displaying and Recording Paths of Motion," there is described a system for providing a television display which portrays the motion of an object in a scene during a specific time interval such that, in addition to its present position being shown, a plurality of earlier positions commencing with the start of the time interval, are also depicted. The system includes a storage device capable of storing one television frame and adapted to accept substitute information concerning individual picture elements of the television signals stored therein in combination with means for comparing the stored television frame with selected subsequently arriving frames. Upon detection of differences between corresponding picture elements of the stored and subsequently arriving frames, signifying motion, the picture elements that caused the detected difference are substituted for corresponding picture elements in the stored frame, and the fact of each such substitution is stored or otherwise indicated. The system is arranged to preclude another substitution of previously substituted picture elements in the stored frame. The frequency of the comparisons, that is, the number of incoming television frames between successive comparisons, is controllable and determines the spacing in the display between successive positions of the moving object.
Although the system of the copending application provides an acceptable display of successive positions of the moving object when the subsequently arriving frames selected for comparison with the stored frame are spaced sufficiently that successive images of the moving object are not contiguous or overlapping, in situations where it is desired to display the total motion of a person, swinging a golf club, for example, if there is any tendency for overlap between successive images (e.g., the person's arms), the latest image is partially occluded because the system gives precedence to the earlier images.
The reasons for this sometimes undesirable characteristic of the earlier system will be seen from analysis of the operation of the system illustrated in FIG. 1 (which corresponds to FIG. 2 of the aforementioned co-pending application). The earlier system includes a pair of multiplexers 14 and 16 each having two inputs designated a and b, and a control terminal c which when enabled is operative to select which of the two inputs is coupled to its output terminal d, a comparator 18, and a switch 20 having an open position R and a closed position S. The incoming video signal is applied to one input terminal of comparator 18 and also to the lower input terminal b of multiplexer 16. The output terminal d of multiplexer 16 is connected to the lower input terminal b of multiplexer 14, and when switch 20 is in the "S" position, is also connected to the input of update frame delay 12, the output of which is connected to the upper input terminal a of multiplexer 16. The output terminal d of multiplexer 14 is connected to the input of reference frame delay 10, the output of which is connected to the upper input terminal a of multiplexer 14, and also to the second input terminal of comparator 18. A control signal at terminal 22 is applied to the control terminal c of both multiplexers and causes them both simultaneously to select either the signal appearing at input terminal a or at the lower input terminal b. Because of the common control of the two multiplexers, all of the update information, representing successive positions of the moving object, is eventually transferred into the reference frame delay 10 and, in one mode of operation, upon operating a "stop" button at the end of the selected "action interval," is coupled to the output along with the reference frame.
Because the c controls of multiplexers 14 and 16 are ganged together, making them operative simultaneously at all times, the reference frame delay 10, which provides one input to comparator 18, is not an immutable reference against which to compare the incoming video signal. That is, although reference frame delay 10 initially contains only the reference background, as the moving object progresses across the scene information concerning the moving object is accumulated in the reference frame; the information concerning the moving object added to the reference frame delay causes the initial "clean" condition to be obscured. The result is that if successive images of the moving object have overlap, the earlier image will effectively lock out the later image from the same portion of the raster because the signal Z will be a "1" which, according to the logic of the prior system, will preclude transferring to the reference frame update information concerning the overlapping portion of the later image. The effect on the displayed image is illustrated in the upper portion of FIG. 2, which depicts successive overlapping images of a ball moving from left to right. It will be seen that in all of the successive positions, the earlier image takes precedence over the next successive image, obscuring the next successive image in the area of overlap, an undesirable result from the viewer's point of view.